This invention relates to signal power level regulation in a satellite or other wireless relayed communications network. In a satellite communication environment, power control is the process in which interactive terminals adjust their EIRP such that all transmissions arrive at the satellite and/or the destination receiver at an appropriate signal level. As uplink fades occur, for example as caused by excessive moisture in the signal path caused by rain or other weather, the power control system causes the interactive terminal's EIRP to increase such that the signal still arrives at the satellite at the desired level. If the interactive terminal runs out of EIRP to compensate for the rain fade, then the interactive terminal can reduce its data rate and set its EIRP such that the resulting Eb/No at the satellite is the same as the Eb/No for all other interactive terminals. This dual control of both EIRP and data rate is more accurately termed an energy control system, although the term power control is generally used in the industry.
FIGS. 1 and 2 illustrate an exemplary satellite environment used to describe the subject invention. In FIG. 1 showing a satellite communication system 100, a hub 101 transmits via a forward uplink transmission 120 to a communications relay satellite 102, which then re-broadcasts the transmission to a number of subscriber terminals (STs) ST111-ST118 over a forward downlink signal 122. In general, the hub 101 broadcasts continuously and is also able to receive its own forward downlink 122.
FIG. 2 depicts the return transmissions of satellite communication system 100. Here, each subscriber terminal ST111-ST118 transmits a return uplink signal to satellite 102 on the return uplink 132. The satellite 102 then retransmits the return signals to the hub 101 using return downlinks 130. The return transmissions are typically sporadic and/or bursty in nature, and often the subscriber terminals ST111-ST118 cannot receive their signal on their own return downlink 130.
FIGS. 1 and 2 depict an example wherein some of the communication transmissions experience propagation degradation, including weather-induced fade, which is illustrated here as an example. There are many other sources of transmission degradation, for example, a mobile subscriber terminal could be deployed under foliage that degrades transmission.
In FIG. 1, a light rain attenuates the forward uplink 120. This attenuation will affect the power received at the satellite 102 and thus the power eventually relayed to all subscriber terminals ST111-ST118. The forward downlink 122 is also degraded for some subscriber (user) terminals. In this example, subscriber terminals ST111 and ST112 are under heavy rain and thus experience further severe signal degradation. Subscriber terminals ST113 and ST114 are under light rain and thus experience further moderate signal degradation. Subscriber terminals ST115 and ST116 are under cloud cover and thus experience further minimal signal degradation. Subscriber terminals ST117 and ST118 are under clear sky and thus experience no further signal degradation.
In FIG. 2, the return uplink 132 is attenuated by the same conditions that affect the forward downlink 122 of FIG. 3. Subscriber terminals ST111 and ST122 are under heavy rain and thus their return uplinks 132 experience severe signal degradation. Subscriber terminals ST113 and ST114 are under light rain and thus their uplinks 132 experience moderate signal degradation. Subscriber terminals ST115 and ST116 are under cloud cover and thus their uplinks 132 experience minimal signal degradation. Subscriber terminals ST117 and ST118 are under clear sky and thus their uplinks 132 experience no signal degradation. Thus, in an uncompensated system, the return uplink signals 132 are received at satellite 102 with widely varying power levels due to the varying transmission conditions. The return downlink signals 130 all receive further degradation (notably the same degradation for all return downlink signals) due to the light rain at the hub 101.
In general, the forward and return channels, and even the uplinks and downlinks, can be on different frequencies. These different frequencies may be attenuated differently by the same channel conditions. For the description of the subject invention, the assumption will be made that any such differences in attenuation can be compensated for.
In the satellite communication industry, much effort has been expended on the problem of power control. For example, “Uplink Power Control Techniques for VSAT Networks,”Thomas Saam, IEEE Southcon 1989, pgs. 96-101. Therein the author describes a technique wherein the hub receives its own signal and removes half of the attenuation to keep the signal level constant at the satellite, a technique described further in U.S. Pat. No. 4,941,199, now commonly owned with the subject invention. The author off the IEEE article also discusses five general categories for transmit power control: Static margin, independent control, centralized control, pilot control, and pair control. Static margin is basically null power control: the power level is set so that there will always be a valid signal received. The excess signal during clear sky conditions is wasted, and the network capacity is thus reduced. Under independent control, subscriber terminals monitor their own conditions and compensate locally. Under centralized control, a central controller broadcasts uplink fade estimates to the other terminals. Under pilot control, each subscriber terminal monitors a beacon or pilot to estimate its uplink fade. Under pair control, subscriber terminals exchange fade estimates with each other.
In the past power control has typically been implemented via a satellite beacon. A terminal receives a beacon signal from the satellite and determines the signal strength. An uplink power control unit uses this beacon signal strength estimate to adjust the power of the terminal's transmission to exactly compensate for the current attenuation of the signal. This technique works very well and is appropriate for high value, high cost terminals, such as a hub. For the subscriber terminals, however, which are typically relatively low-cost, of limited capability, and often very compact, this technique has severe limitations, such as a need for an extra (beacon) receiver, a precision uplink power control attenuation system, and calibration equipment, as examples.
FIG. 3 shows a subset of the components of a hub 101 useful to describe previously employed processes of power control on the forward uplink and the return uplink. The forward data is modulated by the forward modulator 230, which is then typically interfaced to an uplink power control unit 220 via an IF link. (Typical IF frequencies for such an interface are 50-180 MHz or 950-1450 MHz). The modulator 230, shown here as a single unit, may comprise a number of units. For example, if a spread spectrum multiplexing scheme were employed on the forward channel, then a number of modulators with different sources could be summed together to create a composite forward signal. Likewise, the forward modulator 230 may be used to deliver messages from the hub 101 to all subscriber terminals, as will be described hereinafter in connection with a description of the invention.
The power control unit 220 applies a variable attenuator to its input signal (or signals) to create a power compensated signal. The power compensated signal is then transmitted via the transceiver 200 and antenna 250, resulting in a forward uplink signal that is received by the communication relay satellite 102. From the satellite 102 to the antenna 250 is the return downlink (from the subscriber terminals) and a beacon downlink (from the satellite). The forward downlink (not shown) of the hub 101 can operate as the beacon downlink under some conditions.
The received signals at the antenna 250 are then processed by the receiver portion of the transceiver 200 at the hub 101 and passed along (typically at IF) to a beacon receiver 210 and to a return demodulator 240, which provides the return data to the hub. The return demodulator 240 may be a multi-channel demodulator, as in the case of multiple simultaneous return transmissions, such as those encountered in a spread spectrum multiple access system.
The beacon receiver 210 is intended to accurately determine the amplitude of the received beacon and to pass this signal strength to the uplink power control unit 220 to enable the uplink power control unit to create an amplitude-compensated signal. Generally, such beacon receivers are calibrated under clear sky or minimum signal degradation conditions. Thus, any drop in power in the received beacon signal can be attributed to a fade condition on the beacon downlink signal, which corresponds to a fade in the forward uplink signal. This uplink fade is compensated for by the uplink power control unit 220.
FIG. 3 further illustrates internal components of the hub 101 in the application of power control to the return uplink. The received return downlink and the beacon downlink signals at the transceiver 200 again feed the beacon receiver 210 and the return demodulator 240. An estimate of the quality of each subscriber terminal transmission is made in a quality estimator 260. A typical metric of signal quality is the energy per bit divided by the noise density, also known as Eb/N0. This metric can be used to predict the bit error rate performance of the channel and other useful performance attributes, based on well known detection and estimation theory concepts. Another example metric is a direct estimate of the bit error rate, perhaps coming from an error correcting decoder. In general, these signal quality estimates cannot provide an accurate measure of the return signal level at any one point of interest, for example, at the satellite receiver.
In this example hub of FIG. 3, a network management system 270 sends messages to each terminal via the forward modulator 230 in order to direct each user terminal to an appropriate power level, data rate, center frequency, and/or modulation type to maintain the desired signal quality at the hub receiver. This method is generally useful if the absolute power of the return signals is not of concern.
What is needed, therefore, is a method for satellite or other relayed communication system power control which does not require additional or expensive components and calibration of the subscriber terminals and which can also provide absolute power level control.